The reciprocating pump is a specific type of positive displacement pump in which a constant and fixed volume of fluid is drawn into a cylinder by a retreating piston and then discharged under pressure. Its chief virtue compared with other types of positive displacement pumps such as vane, lobe, and screw pumps is that because of the good seal typically achievable between a piston and its cylinder wall, back-leakage of fluid around the piston is low compared to back-leakage of fluid in the other types. This is because a piston and its cylinder meet along a two-dimensional cylindrical surface, whereas a pump vane, screw, or lobe contacts its casing along a line. Thus, reciprocating pumps can deliver very high pressures with relatively high efficiency.
Typically the pistons in a reciprocating pump are driven by connecting rods that are in turn driven by a crankshaft. The connecting rods convert the rotational motion of the crankshaft into linear reciprocal motion of the piston. However, the connecting rods are only directly in line with the motion of the pistons at the top and bottom of the piston travel within the cylinder. Thus, along the rest of the path of the piston, the connecting rods are applying a sidewards force to the piston against the cylinder wall. This generates heat, wear, and pulsating stress on the pumping parts. The heat represents a loss of efficiency. These effects increase as the pressure rating of the pump is increased, that is, particularly in instances where such a pump is the pump of choice.